This invention deals generally with mechanical transmissions and more specifically with a power shift transmission with a large number of forward ratios and very smooth shifts between gear ratios.
Power shift transmissions have been in use for agricultural tractors for about 40 years. Such transmissions now provide the capability of shifting through all the forward gears while moving and while under load without using a clutch pedal. The only action required by an operator is the selection of the desired gear. The actual changeover, including the operation of one or more clutches is electronically controlled and performed by hydraulically powered clutches.
In order to provide a wide range of ratios divided into a large number of small forward ratio steps in a transmission of practical size and reasonable cost, such transmissions are actually built as two or three transmissions in a series arrangement. Such a transmission is disclosed in U.S. Pat. No. 5,036,718 issued to Bulgrien. However, in such transmissions some of the shifts between adjacent gear ratios require complex simultaneous changeovers in two or three of the transmissions. These complex changeovers frequently result in jerky shifts. Moreover, the tendency to increase the spread of the overall ratio over the years has increased the number and severity of these difficult shifts.
For example, considering only forward speeds, the 18 speed tractor transmission mentioned above is constructed with an first section having a three speed transmission with approximately 1.17 ratio steps. The first section is followed by a middle section that is a three speed transmission with ratio steps of approximately 1.6 and then an output section that is a two speed transmission with a 4.1 ratio step. First gear is achieved by using the lowest ratio in each of the three transmission sections. Second and third gears are then achieved by shifting only the input section while leaving the lowest ratio selected in the middle and output sections. When shifting from 3rd to 4th gear, the middle section is shifted from its lowest ratio to its intermediate ratio while the input section is shifted from its highest ratio to its lowest ratio. This multiple changeover pattern continues through all of the ratio combinations to yield the 18 different gear ratios.
However, such a transmission causes discontinuities in the vehicle motion, the so called jerky shifts, when one or more sections are being upshifted while another is being shifted down. Each individual shift involves clutch action that engages one clutch and gear set and disengages another. Such changeovers are referred to in the industry as “clutch swaps”. The multiple gear changes are then referred to as “double swaps” and “triple swaps” as each section of the transmission is shifted by “swapping” clutches, that is, releasing one clutch and applying another. Shifting two sections thus involves two clutch changeovers, a double swap, and shifting all three sections involves three changeovers, a triple swap.
The worst condition for the transmission described above occurs when making the triple swap required to downshift from 10th to 9th gear while under load. To accomplish this shift, the input and middle sections are both shifted from their lowest ratios to their highest ratios while the output section is shifted from its high ratio to is low ratio. To make this shift with minimal change in ground speed would require setting the pressure in the high ratio clutch in the output section to a level that is just sufficient to transmit the torque required by the load, then shifting the input and middle sections to bring up the speed of the intermediate gears and shafts, then completing the shift to the low ratio clutch in the output section. To do this the engine and flywheel would have to momentarily provide enough torque to pull a 10th gear load in 18th gear while the intermediate gears and shafts accelerate. 18th gear is about 3.5 times faster than 10th gear. In fact, the clutches in the input and middle sections do not have sufficient torque capacity to do this, and even if they did the loss in engine speed would be severe. To complete the shift without coming to a complete stop and without excessive loss of engine speed, the output speed is allowed to drop rapidly for a brief time while the intermediate gears and shafts accelerate. It is this action that causes a jerky shift.
The ratio change in the transmission sections that are shifted up during an overall downshift is a good indicator of the difficulty in making shifts involving multiple clutch swaps. In the transmission described above, the shift from 7th to 6th gear can be made quite smoothly. This shift requires shifting the input section from its lowest ratio to its highest ratio while shifting the middle section from its highest ratio to its intermediate ratio. To make this shift with minimal change in output speed requires effectively momentarily pulling a 7th gear load in 9th gear. The ratio between 7th gear and 9th gear is only about 1.37 compared to the ratio of 3.5 between 10th gear and 18th gear discussed in the example above, so the downshift from 7th gear to 6th gear is much smoother and less difficult than the shift from 10th gear to 9th gear.
One very beneficial solution would be to have a power shift transmission in which all shifts between adjacent gears were accomplished with single clutch swaps as described in U.S. Pat. No. 6,190,280 to Horsch, because this would theoretically provides smooth shifts going both up and down. However, the Horsch transmission has rather uneven ratio steps and this condition is further aggravated if the overall ratio range is increased.